In his youth, Kirt Niedrigh applied for membership in the Legion of Super-Heroes and called himself Absorbancy Boy. Kirt had the power to absorb the powers of any metahuman being, but was rejected because his powers were limited. Further, Saturn Girl had analyzed his mind, and discovered his tendencies toward evil.
Angered by being rejected, Kirt found the "Zoraz" costume, which the Legionnaires used as a test for new Legionnaires. As "Zoraz", Kirt attacked the Legionnaires. After defeating them, Kirt revealed his identity, believing he had earned his place. The Legion did not agree, and Superboy attempted to take the suit from him. Kirt used the suit to project red sun radiation at Superboy, and absorbed his powers. He defeated Superboy, but Kirt himself was beaten by Legion applicant Tyroc.
The being known as the Time Trapper planted a crystal tablet in the Arctic that faked evidence that Superman was from Earth and not Krypton. Later, Kirt became known as Earth-Man, and he had claimed to have discovered the crystal artifact in the Arctic which proved that Superman was not an alien, but a human who was given abilites to protect Earth from alien invaders. Earth-Man used this information to cause distrust of aliens among the Earth people. Earth-Man was sucessful, as Earth seceded from the United Planets. All remaining aliens living on earth were deported or sent to prison camps. Other human Legion rejects joined Earth-Man's cause, including Spider Girl, Golden Boy, Storm Boy, Eyeful Ethel, Tusker and Radiation Roy. They formed the Justice League of Earth, a group dedicated to uphold Superman's beliefs. Many Legionnaires went on the run or captured by the Justice League. Earth-Man kept many Legionnaires held prisoner in the Justice League's satellite headquarters, and he absorbed their powers for himself. Using Sun Boy, he turned Earth's yellow sun into a red one.
Brainiac 5 sent for Superman, and brought him to the 31st century through the use of a Time Bubble. Together with other free Legionnaires, and the Legion of Substitute Heroes, they attacked the Justice League's satellite headquarters. Earth-Man took on Superman one on one, and with Superman powerless, was beating him easily. However, Brainiac 5 freed Sun Boy, and with Earth's sun yellow, Superman could fight Earth-Man equally. With Superman's powers returned, and the people of Earth viewing Earth-Man's actions, it convinced the people of Metropolis that they had been fooled. Earth-Man was doing well against Superman, but when all the freed Legionnaires arrived, he did not have a chance. Earth-Man and the Justice League of Earth were sent to Takron-Galtos.
The JLE, along with Earth-Man later joined forces with the Legion of Super-Villains, Fatal Five, Universo and Mordru. Since they had the same goal of destroying the Legion, Earth-Man agreed to join with them temporarily. With the combined might of the Legions of 3 Worlds (Earth-0, Earth-247 and Earth Prime) Superboy and Kid Flash, the villains were defeated and Earth-Man was sent back to prison.
Earth-Man still had pro-human supporters, and the government of Earth forced the Legion to accept Earth-Man into his ranks or be forced to leave Earth. Begrudgingly, the Legion offered Earth-Man membership, however, Brainiac 5 put certain 'controls' in Earth-Man's Legion Flight Ring. As Earth-Man was considering the Legion's offer, the being known as Dyogene of Oa, offered Earth-Man a Green Lantern Power Ring so that Earth-Man could be the new Green Lantern of sector 2814. Earth-Man accepts both offers, but uses his power ring to secretly disable most of the failsafes the Brainiac 5 put on his flight ring.
Earth-Man's first mission was to help the Legion in stopping an anti-alien riot. Back at headquarters, he asked the ring how it works. The ring told him, and produced the power battery for him to recharge. Earth-Man then revealed his power ring to Colossal Boy, while the power ring was prompting him to rescue sentients on the planet Ozifer. Earth-Man tried to aid the planet, but was overcome by the native creatures. He is saved by Sun Boy, and, with Element Lad's help, restores Ozifer's atmosphere to normal. Earth-Man throws the power ring into the swamp, and said he would not be a slave to it. He then journeyed back to Earth with the Legionnaires.
Earth-Man was later confronted by pro-human supporters, accusing him of not supporting their cause. Earth-Man assured them that he was going to save the Earth his way, and for them not to contact him again.
When the Legion face off against an ancient being known as the Bringer of Chaos, Earth-Man absorbs the powers of both his Legionnaires and the villain himself. Earth-Man kills the Bringer of Chaos, but dies himself in the confrontation. Shadow Lass takes his body back to Earth, proclaiming him to be a true champion